Composite particles for electrochemical element electrode

ABSTRACT

The present invention provides a method of producing composite particle for high density electrochemical element electrodes in electrochemical elements having low internal resistance and high capacitance. 
     Slurry containing an electric conductive material and a binder is obtained, and the slurry is sprayed to a fluidized electrode active material to carry out fluidized-granulation, and further particles obtained by the fluidized-granulation are rolling-fluidized granulated, and thereby, composite particle for electrochemical element electrode, containing electrode active materials, electric conductive materials, and binders, and being structured of an outer layer portion (outer shell portion) and an inner layer portion (core portion), where the outer shell portion and the core portion are made by bonding the electrode active material and the electric conductive material by the binder, and the weight average particle diameter of the electrode active material and the electric conductive material which form the outer shell portion is smaller than the weight average particle diameter of the electrode active material and the electric conductive material which form the core portion are obtained.

TECHNICAL FIELD

The present invention relates to composite particles which constitute anelectrode material used preferably for electrochemical elements such asa lithium ion secondary battery and an electric double layer capacitor,in particular an electric double layer capacitor (in the presentspecification, referred to simply as “composite particles”) Further, thepresent invention relates to an electrode material comprising thecomposite particles, and an electrochemical element electrode using theelectrode material.

BACKGROUND ART

Electrochemical elements such as a lithium ion secondary battery and anelectric double layer capacitor have advantageous characteristics thatthey are small, lightweight, their energy density is high, and they canbe repeatedly charged and discharged, and accordingly their demand isexpanding rapidly. The lithium ion secondary battery has a relativelylarge energy density, and it is used in the fields such as cellularphones and notebook personal computers, meanwhile the electric doublelayer capacitor can be rapidly charged and discharged, and it is used asa memory backup small power source in personal computers and the like.Furthermore, the electric double layer capacitor is expected to beapplied as a large sized power source for electric vehicles. Moreover,the redox capacitor using the oxidation-reduction reaction (pseudoelectricity double layer capacitance) on the surface of a metal oxide ora conductive polymer also attracts attention because of the size of itscapacitance. As for these electrochemical elements, along with theexpansion of their applications, further more improvements are requiredfor lower resistance, higher capacitance, more excellent mechanicalproperties and the like. Under such circumstances, in order to enhancethe performances of electrochemical elements, various improvements arealso made in the materials which form electrochemical elementelectrodes.

The electrochemical element electrodes are, in general, made bylaminating active material layers which are formed by bonding electrodeactive materials such as activated carbon and lithium metal oxide, andelectric conductive materials on a collector.

Patent Documents 1 and 2 disclose a method of pressurizing and formingcomposite particles obtained by bonding particulate electrode activematerials and particulate electric conducting auxiliary agents by abinder to obtain active material layers. The composite particles used inthe Patent Documents 1 and 2 have the structure where particulateelectrode active materials and particulate electric conducting auxiliaryagents are distributed uniformly in the composite particles as shown inFIG. 1. However, the composite particles have inferior formability, andaccordingly, it has been difficult to obtain electrode sheets stably andcontinuously.

Further, Patent Documents 3 discloses a method where a slurry mixedmaterial containing electrode active materials, thermosetting resin, andsolvent is formed, and this mixed material is granulated by the spraydry method to obtained composite particles, and the composite particlesare fixed by the hot pressing, roll pressing, or other means on acollector to form active material layers. The particles obtained in thePatent Documents 3 are, as shown in FIG. 2, hollow particles having thehusks formed with bonded particulate electrode active materials.

However, in the electrodes formed of these particles, the density of itsactive material layers is low, and accordingly, only electrochemicalelements with small capacitance have been obtained.

-   [Patent Document 1] Japanese Unexamined Patent Application    Publication No.2005-78943-   [Patent Document 2] United States Patent Publication No.    2005/0064069-   [Patent Document 3] Japanese Unexamined Patent Application    Publication No. H09-289142

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

The object of the present invention is to provide composite particlesfor electrochemical element electrodes for obtaining electrochemicalelements having both low internal resistance and high capacitance,electrode materials comprising the composite particles, and electrodesformed by the electrode materials.

Means to Solve the Problems

The present inventors have made examinations wholeheartedly in order toattain the above object, as a result, the present inventors have foundthat the internal resistance of the electrochemical element becomes lowand the capacity becomes large and the electrode density becomes high,by forming an active material layer on a collector using electrodematerials comprising composite particles for electrochemical elementelectrodes which comprises electrode active materials, electricconductive materials and binders, and being structured of an outer layerportion (outer shell portion) and an inner layer portion (core portion),where the outer shell portion and the core portion are made by bondingthe electrode active materials and the electric conductive materials bythe binders, and the weight average particle diameter of, the electrodeactive material and the electric conductive material which form theouter shell portion, is smaller than the weight average particlediameter of, the electrode active material and the electric conductivematerial which form the core portion. The present inventors have come tocomplete the present invention on the basis of the above findings.

According to the present invention, there are provided compositeparticles for electrochemical element electrodes, comprising electrodeactive materials, electric conductive materials, and binders, and beingstructured of an outer layer portion (outer shell portion) and an innerlayer portion (core portion), where the outer shell portion and the coreportion are made by bonding the electrode active material and theelectric conductive material by the binder, and the weight averageparticle diameter of the electrode active material and the electricconductive material which form the outer shell portion is smaller thanthe weight average particle diameter of the electrode active materialand the electric conductive material which form the core portion.

Moreover, according to the present invention, there are provided anelectrochemical element electrode material comprising the abovecomposite particles for electrochemical element electrodes, and anelectrochemical element electrode where an active material layer made ofthe electrochemical element electrode material is laminated on acollector.

Furthermore, according to the present invention, there are provided amethod of producing composite particles for electrochemical elementelectrodes comprising steps of:

obtaining slurry A containing an electric conductive material and abinder,

fluidizing an electrode active material and fluidized-granulating byspraying the above slurry A thereto, and

rolling-fluidized granulating the particles obtained in thefluidized-granulating step in the presense of slurry A; and

a method of producing composite particles for electrochemical elementelectrodes comprising the steps of:

obtaining slurry B containing an electrode active material, an electricconductive material and a binder, and

spray-granulating by spray-drying the above slurry B with a pin typeatomizer.

EFFECTS OF THE INVENTION

Composite particles for electrochemical element electrodes according tothe present invention have a structure where an electrode activematerial and an electric conductive material (mainly electric conductivematerial) whose weight average particle diameter is comparatively smallare distributed in the outer layer portion (outer shell portion), and anelectrode active material and an electric conductive material (mainlyelectrode active material) whose weight average particle diameter iscomparatively large are distributed in the inner layer portion (coreportion). In the electrode obtained using this composite particle, theinternal resistance becomes low. Moreover, the core portion is occupiedby particles having a large particle diameter, and accordingly, a lot offine pores are distributed, and it has a structure where electrolysissolution permeates easily, and large electric capacitance can beobtained. Further, the electrode for electrochemical elements obtainedusing the electrochemical element electrode material comprising thecomposite particles can be used for electrochemical elements which canperform storage and conversion of energy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing a composite particle of the priorart.

FIG. 2 is a sectional view showing a composite particle of the priorart.

FIG. 3 is a sectional view showing an example of a composite particle ofthe present invention.

FIG. 4 is a figure showing an example of a method of producing anelectrode.

FIG. 5 is a figure showing an example of the spray-drying apparatus usedin the present Example.

FIG. 6 is a figure showing the electron microscope observation image ofthe cross sectional view of a composite particle obtained in Example 1.

BEST MODE FOR CARRYING OUT THE INVENTION

Composite particle for electrochemical element electrode according tothe present invention has a structure comprising electrode activematerial, electric conductive material, and binder, and being structuredof an outer layer portion (outer shell portion) and an inner layerportion (core portion), where the outer shell portion and the coreportion are made by bonding the electrode active material and theelectric conductive material by the binder, and the weight averageparticle diameter of the electrode active material and the electricconductive material which form the outer shell portion is smaller thanthe weight average particle diameter of the electrode active materialand the electric conductive material which form the core portion.

The electrode active material which makes up the composite particle ofthe present invention is suitably chosen according to the type ofelectrochemical elements. As the electrode active material for thepositive electrode of a lithium ion secondary battery,lithium-containing compound metal oxides such as LiCoO₂, LiNiO₂, LiMnO₂,LiMn₂O₄, LiFePO₄, LiFeVO₄; transition metal sulfides such as TiS₂, TiS₃,amorphous MoS₃; transition metal oxides such as Cu₂V₂O₃, amorphousV₂O.P₂O₅, MoO₃, V₂O₅, V₆O₁₃; may be exampled. Furthermore, conductivepolymers such as polyacetylene and poly-p-phenylene may be mentioned.

As the electrode active material for the negative electrode of a lithiumion secondary battery may be mentioned, for example, carbonaceousmaterials such as amorphous carbon, graphite, natural graphite, mesocarbon micro bead (MCMB), and pitch based carbon fiber; conductivepolymers such as polyacene. These respective electrode active materialsmay be used alone or in combination of two or more, according to thetype of electrochemical elements. When the electrode active materialsare used in combinations, two or more kinds of electrode activematerials having different average particle diameters or particlediameter distributions may be used in combination.

It is preferable that the shape of the electrode active material usedfor the electrode of a lithium ion secondary battery is granulated intospherical particles. If the shape of particles is spherical, a highdensity electrode can be formed when the electrode is formed. Moreover,mixture of fine particles with an average particle diameter of about 1μm and comparatively large particles with an average particle diameterfrom 3 to 8 μm, or particles having a broad particle diameterdistribution from 0.5 to 8 μm are preferable. It is preferable that incase of use, particles with diameter of 50 μm or larger are eliminatedby screening and the like. Although the tap density specified by ASTMD4164 of electrode active material is not restricted in particular, oneof 2 g/cm³ or more is used preferably in the positive electrode, and oneof 0.6 g/cm³ or more is preferably used in the negative electrode.

As the electrode active material for electric double layer capacitors,usually, a carbonaceous material is used. It is preferable that theelectrode active material for electric double layer capacitors has alarge specific surface area enough to form an interface of a larger areaeven with a same weight. In particular, it is preferable that thespecific surface area is 30 m²/g or more, preferably from 500 m²/g to5,000 m²/g, further preferably from 1,000 m²/g to 3,000 m²/g. Asspecific examples of the carbonaceous materials, activated carbon,polyacene, carbon whisker, graphite, and the like may be mentioned, andpowder or fibers of these can be used. A preferable electrode activematerial for electric double layer capacitors is activated carbon, andspecifically, activated carbon made from phenol, rayon, acryl, pitch, orcoconut husk may be listed up. As the electrode active material forelectric double layer capacitors, these respective carbonaceousmaterials may be used alone or in combination of two or more. When thecarbonaceous materials are used in combination, two or more kinds ofcarbonaceous materials having different average particle diameters orparticle diameter distributions may be used in combination.

Moreover, non-porosity carbon that has micro crystallite carbon similarto graphite and the interlayer distance between layers of the microcrystallite carbon is expanded may be used as the electrode activematerial. Such non-porosity carbon is obtained by dry-distillinggraphitizable carbon with developed micro crystallite of multilayergraphite structure from 700° C. to 850° C., subsequently heat treatingit with a caustic alkali from 800° C. to 900° C., and further removingresidual alkali ingredient with heated steam at necessity.

As the electrode active material for electric double layer capacitors,when powder whose weight average particle diameter is from 0.1 μm to 100μm, preferably from 1 μm to 50 μm, further preferably from 5 μm to 20 μmis used, it is possible to easily make thin film-shape electrode forelectric double layer capacitors, and to make the electric capacityhigh, which is preferable.

The electric conductive material which makes up the composite particlesaccording to the present invention is a particulate carbon materialwhich has conductivity, and does not have fine pores which can form anelectric double layer, and is to increase the conductivity of anelectrochemical element electrode. The weight average particle diameterof the electric conductive material is smaller than the weight averageparticle diameter of the electrode active material, and is in the rangeusually of 0.001 μm to 10 μm, preferably 0.05 μm to 5 μm, furtherpreferably 0.01 μm to 1 μm. When the weight average particle diameter ofthe electric conductive material is in this range, high conductivity maybe provided for the electrochemical element electrode by a smaller usedamount. In particular, conductive carbon black such as furnace black,acetylene black, and KETJEN BLACK (registered trademark of Aczo NobelChemicals B.V.); and graphite such as natural graphite and artificialgraphite; may be employed. Also in these, conductive carbon black ispreferable, and acetylene black and furnace black are furtherpreferable. These respective electric conductive materials may be usedalone or in combination of two or more.

The amount of the electric conductive material to 100 parts by weight ofthe electrode active material is usually in the range of 0.1 part to 50parts by weight, preferably 0.5 part to 15 parts by weight, and furtherpreferably 1 part to 10 parts by weight. By using the electrode whoseelectric conductive material quantity is in this range, it is possibleto make the capacity of an electrochemical element high and make theinternal resistance low.

The binder used in the present invention is not limited in particular solong as it is a compound which has bonding ability, meanwhile adispersible binder is preferable. The dispersible binder is a binderwith the character to be dispersed in a solvent. For example, highmolecular compounds such as fluoride polymer, diene polymer, acrylatepolymer, polyimide, polyamide, polyurethane and so on may be mentioned,and further preferably, fluoride polymer, diene polymer, and acrylatepolymer may be mentioned. These respective binders may be used alone ormay be used in combination of two or more.

The fluoride polymer is a polymer containing a monomer units containinga fluorine atom. The amount of the monomer unit containing the fluorinein the fluoride polymer is usually 50% by weight or more. As examples ofthe fluoride polymer, fluororesins such as polytetrafluoroethylene, andpolyvinylidenefluoride may be used, and polytetrafluoroethylene ispreferable.

The diene polymer is a polymer including a monomer unit derived fromconjugated diene such as butadiene, isoprene and so on and hydrogenatedproduct thereof. The amount of the monomer unit derived from conjugateddiene in the diene polymer is usually 40% by weight or more, preferably50% by weight or more, and further preferably 60% by weight or more.Specifically, conjugated diene homo-polymers such as polybutadiene,polyisoprene; aromatic vinyl-conjugated diene copolymers such as styrenebutadiene copolymer (SBR) which may be carboxy-modified; vinyl cyanideand conjugated diene copolymer such as acrylonitrile butadienecopolymers (NBR); hydrogenated SBR, hydrogenated NBR, and the like maybe listed up.

The acrylate polymer is a polymer including a monomer unit derived fromacrylic acid ester and/or methacrylic acid ester. The amount of themonomer unit derived from acrylic acid ester and/or methacrylic acidester in the acrylate polymer is usually 40% by weight or more,preferably 50% by weight or more, and further preferably 60% by weightor more. As specific examples of the acrylate polymer may be mentionedcross-linked acrylate polymers such as copolymer of 2-ethylhexylacrylate, methacrylic acid, acrylonitrile and ethyleneglycoldimethacrylate, copolymer of 2-ethylhexyl acrylate, methacrylic acid,methacrylonitrile and diethyleneglycol dimethacrylate, copolymer of2-ethylhexyl acrylate, styrene, methacrylic acid and ethyleneglycoldimethacrylate, copolymer of butyl acrylate, acrylonitrile anddiethyleneglycol dimethacrylate, and copolymer of butyl acrylate,acrylic acid and trimethylolpropane tri-methacrylate; copolymers ofethylene and acrylic (or methacrylic) acid ester such as copolymer ofethylene and methyl acrylate, copolymer of ethylene and methylmethacrylate, copolymer of ethylene and ethyl acrylate, and copolymer ofethylene and ethyl methacrylate; and graft polymers where a radicalpolymerizable monomer is grafted to the above copolymer of ethylene andacrylic (or methacrylic) acid ester. Note that, as the radicalpolymerizable monomer used for the above graft polymers, methylmethacrylate, acrylonitrile, methacrylic acid, and the like may bementioned. In addition, copolymer of ethylene and acrylic (ormethacrylic) acid such as copolymer of ethylene and acrylic acid,copolymer of ethylene and methacrylic acid and the like may be used asthe binder.

Among these, the diene polymer and the cross-linked acrylate polymer arepreferable, and the cross-linked acrylate polymer is in particularpreferable, from the viewpoints that they can obtain an active materiallayer excellent in bonding property with a collector and surfacesmoothness, moreover, they enable to produce electrodes forelectrochemical elements with high electric capacity and low internalresistance.

Although the binder used for the present invention is not limited inparticular as for the shape, it is preferable that the binder isparticulate since its bonding property is good, and the decline of theelectric capacity of produced electrodes and the deterioration thereofby the repetition of charge and discharge can be restrained. As theparticulate binder, for example, one in the state where the particles ofthe binder are dispersed in the solvent like Latex, and powdered oneobtained by drying such dispersed liquid may be employed.

Moreover, the binder used in the present invention may be particleswhich have a core-shell structure obtainable by stepwise-polymerizingtwo or more kinds of monomer mixtures. It is preferable that the binderwhich has the core-shell structure is obtained by first polymerizing themonomer that gives the first polymer to obtain a seed particle, andpolymerizing the monomer that gives the second polymer in the presenceof the seed particle.

Although the ratio or the core and the shell of the binder having theabove core-shell structure is not limited in particular, the ratio ofcore portion:shell portion in mass is usually from 50:50 to 99:1,preferably from 60:40 to 99:1, and further preferably from 70:30 to99:1. The high molecular compound which makes up the core portion andthe shell portion may be chosen from the above high molecular compounds.As for the core portion and the shell portion, it is preferable that oneof these has a glass transition temperature below 0° C., and, the otherhas a glass transition temperature of 0° C. or higher. Moreover, thedifference of the glass transition temperature of the core portion andthe shell portion is usually 20° C. or higher, and preferably 50° C. orhigher.

Although the average particle diameter of the particulate binder used inthe present invention is not limited in particular, it is usually from0.0001 μm to 100 μm, preferably from 0.001 μm to 10 μm, and furtherpreferably from 0.01 μm to 1 μm. When the average particle diameter ofthe binder is in this range, it is possible to give excellent bondingforce even by use of a small amount of binder to the active materiallayer. Herein, the average particle diameter is the number averageparticle diameter which is calculated as the arithmetic average value ofmeasured diameters of 100 binder particles chosen at random by use of atransmission electron microscope photograph. The shape of the particlesmay be either spherical or irregular.

The used amount of this binder to 100 parts by weight of the electrodeactive material is in the range of usually 0.1 to 50 parts by weight,preferably 0.5 to 20 parts by weight, and further preferably 1 to 10parts by weight.

As for the composite particles according to the present invention, whena dispersible binder is used as the binder, it is preferable that itfurther contains soluble resin. This soluble resin is a resin whichdissolves in solvent and further preferably has the function to assistthe electrode active material, the electric conductive material, and thelike to be dispersed uniformly in the solvent. The soluble resin mayhave or may not have bonding ability. As the soluble resin may bementioned cellulose polymers such as carboxymethyl cellulose, methylcellulose, ethyl cellulose and hydroxypropyl cellulose, and ammoniumsalt or alkaline metal salt of these; salts of poly acrylic (ormethacrylic) acid such as poly sodium acrylate (or methacrylate);polyvinyl alcohol, modified polyvinyl alcohol, polyethylene oxide; polyvinyl pyrrolidone, polycarboxylic acids, starch oxide, starch phosphate,casein, various denatured starches, a chitin, chitosan derivatives, andthe like. These respective soluble resins may be used alone or may beused in combination of two or more. In particular, cellulose polymersare preferable, and carboxymethyl cellulose or its ammonium salt oralkaline metal salt is in particular preferable. Although the usedamount of the soluble resin is not limited in particular, the usedamount to 100 parts by weight of the electrode active material is in therange of usually 0.1 to 10 parts by weight, preferably 0.5 to 5parts byweight, and further preferably 0.8 to 2 parts by weight. By using thesoluble resin, sedimentation and condensation of the solid contents inslurry can be restrained. Moreover, since clogging of the atomizer atthe moment of spray-drying can be prevented, it is possible to performthe spray-drying stably and continuously.

The composite particles according to the present invention may containfurther other additives at necessity. As other additives, for example,there are surface active agents. As the surface active agents arementioned anionic surface active agents, cationic surface active agents,nonionic surface active agents, and ampholytic surface active agentssuch as nonionic-anionic surface active agents, meanwhile, anionic ornonionic surface active agents that are easily thermally decomposed areespecially preferable. The used amount of the surface active agent isnot limited in particulate, but the used amount thereof to 100 parts byweight of the electrode active material is in the range of 0 to 50 partsby weight, preferably 0.1 to 10 parts by weight, and further preferably0.5 to 5 parts by weight.

The composite particles according to the present invention arestructured of an outer layer portion (outer shell portion) and an innerlayer portion (core portion), and both of the outer shell portion andthe core portion are made by bonding the above electrode active materialand the electric conductive material by the binder, and the weightaverage particle diameter of the electrode active material and theelectric conductive material which form the outer shell portion issmaller than the weight average particle diameter of the electrodeactive material and the electric conductive material which form the coreportion. FIG. 3 is a figure schematically showing the cross sectionalview of a composite particle 3 according to the present invention.

The outer layer portion (outer shell portion) of the composite particleis formed by bonding electrode active materials 12 and/or electricconductive materials 11 whose weight average particle diameter iscomparatively small. Therefore, it is dense and has few unfilledportions.

On the other hand, the inner layer portion (core portion) of thecomposite particle is formed by bonding electrode active materials 12and/or electric conductive materials 11 whose weight average particlediameter is comparatively large. Since it is formed of the materialswhose weight average particle diameter is comparatively large, unfilledportions between electrode active materials and/or electric conductivematerials are large. That the composite particle is structured of theouter layer portion (outer shell portion) and the inner layer portion(core portion), and the weight average particle diameter of theelectrode active material and the electric conductive material whichform the outer shell portion is smaller than the weight average particlediameter of the electrode active material and the electric conductivematerial which form the core portion can be determined easily byobserving an electron microscopic picture of the cross section of thecomposite particle.

As described above, when electric conductive materials smaller than theelectrode active materials are used, many electric conductive materialsare distributed in the composite particle outer layer portion (outershell portion), and many electrode active materials are distributed incomposite particle inner layer portion (core portion). When manyelectric conductive materials are distributed in the outer shellportion, the conductivity of the surface of the composite particlebecomes high. It is thought that since composite particles contactmutually on the surface when the active material layers are formed, itbecomes easy to pass along electricity, and resistance becomes low.Moreover, it is thought that since there are many unfilled portionswhich pass to many electrode active materials distributed in the coreportion, the migrating of ion becomes well, therefore it is surmisedthat the capacitance becomes high.

The weight average particle diameter of the composite particlesaccording to the present invention is in the range of usually 0.1 μm to1000 μm, preferably 5 μm to 500 μm, and further preferably is 10 μm to100 μm.

Although the producing method of the composite particles forelectrochemical element electrodes according to the present invention isnot limited in particular, the following two producing methods arepreferable, and the composite particles may be easily obtained by these.

The first producing method comprises a step of obtaining slurry Acontaining electric conductive materials, a binder, and soluble resinand other additives to be added at necessity, a step of fluidizing theelectrode active material and fluidized-granulating by spraying theslurry A thereto, and a step of rolling-fluidized granulating theparticles obtained in the above fluidized-granulating step.

First, the slurry A which comprises electric conductive materials, abinder, and soluble resin and other additives to be added at necessityis obtained. As the solvent used in order to obtain the slurry A, ingeneral, water is used preferably, organic solvent may also be used. Asthe organic solvent, for example, alkyl alcohols such as methyl alcohol,ethyl alcohol, propyl alcohol; alkyl ketones such as acetone, methylethyl ketone; ethers such as tetrahydrofuran, dioxane, diglyme; amidessuch as diethyl formamide, dimethyl acetamide, N-methyl-2-pyrrolidone(hereinafter referred to as NMP), dimethyl imidazolidinone;sulfur-containing solvents such as dimethyl sulfoxide, sulfolane; may bementioned. Of these, alcohols are preferable. If an organic solventwhose boiling point is lower than that of water is used in combinationwith water, the drying can be made fast at the time offluidized-granulation. Moreover, since the dispersibility of the binderor the solubility of the soluble resin changes, and the viscosity andflowability of the slurry A may be adjusted by the quantity or the typeof the organic solvent, it is possible to improve the producingefficiency.

The amount of the solvent used in preparing the slurry A is such thatthe solid content concentration of the slurry A is usually in the rangeof 1% to 50% by weight, preferably 5% to 50% by weight, and furtherpreferably 10% to 30% by weight. When the solid content concentration isin this range, the binder disperses uniformly, which is preferable.

The method or procedure to disperse or dissolve the above electricconductive material and the binder, and the soluble resin at necessityin the solvent is not limited in particular. For example, the method ofadding the electric conductive material, the binder, and the solubleresin into the solvent to mix them; the method of dissolving the solubleresin in the solvent and adding the binder (for example, Latex)dispersed in the solvent to mix them, and finally adding the electricconductive material to mix them; the method of adding the electricconductive material into the soluble resin dissolved in the solvent tomix them and adding the dispersible binder dispersed in the solvent intothem to mix them; and the like may be mentioned. As means for mixing,for example, mixer machines such as a ball mill, a sand mill, a beadmill, a pigment dispersion machine, a crushing machine, an ultrasonicdispersion machine, a homogenizer, a planetary mixer may be mentioned.The mixing is usually performed in the range of room temperature to 80°C., and for 10 minutes to several hours.

Next, the electrode active material is fluidized, and the above slurry Ais sprayed thereto, and fluidized-granulation is performed. Thefluidized-granulation method includes a method by a fluidized bed, amethod by a modified fluidized bed, a method by a spouted bed, and thelike. The method by a fluidized bed is one of fluidizing the electrodeactive material by a hot wind, and atomizing the above slurry A to thisfrom a spray and the like to perform agglomeration and granulation. Themethod by a modified fluidized bed is same as the method by a fluidizedbed, except that it is a method of giving a circulation flow to thepowder in the bed, and of taking those granulated matters out which growcomparatively large by using the classifying effect.

Moreover, the method by a spouted bed is one of adhering the slurry Afrom a spray and the like to coarse particles by use of the feature of aspouted bed to dry and granulate them simultaneously. As the process ofthe present invention, among these three methods, the method by afluidized bed or the method by a modified fluidized bed are preferable.

Although the temperature of the slurry to be sprayed is usually at theroom temperature, but it may be warmed to higher than the roomtemperature. The temperature of the hot wind used for fluidization isusually from 80° C. to 300° C., and preferably from 100° C. to 200° C.

Although the particles A obtained by the fluidized-granulation may becompletely dried by a hot wind, meanwhile in order to increase thegranulation efficiency in the following rolling-fluidized granulatingprocess, particles in a wet condition are preferable.

Subsequently, the particles A obtained at the abovefluidized-granulation process is rolling-fluidized granulated in thepresence of the slurry A containing the electric conductive material andthe binder. In addition, if the slurry A used for the rolling-fluidizedgranulation contains the electric conductive material and the binder, itmay be same as or different from the slurry A used in thefluidized-granulation. The rolling-fluidized granulation method includea rotary plate method, a rotary cylinder method, a rotary head cut conemethod and the like. The rotary plate method is a method where the aboveslurry A is sprayed to the particles A supplied in an inclined rotatingplate to generate agglutinated granulated matters, and those granulatedmatters which grow comparatively large are taken out from a rim by useof the classifying effect of the rotary plate. The rotary cylindermethod is a method where wet particles A are supplied to the inclinedrotating cylinder, and rolled in the cylinder, and the above slurry A issprayed thereto to obtain agglutinated granulated matters. The rotaryhead cut cone method is same as the operation system of the rotarycylinder method, except that it is a method where those granulatedmatters which grow comparatively large are taken out by use of theclassifying effect of the agglutinated granulated matters from the headcut cone. In this rolling-fluidized granulation step, coveringgranulation is mainly performed, and agglutinated granulation isperformed partly.

Although the temperature at the time of rolling-fluidized granulation isnot limited in particular, in order to remove the solvent whichconstitutes the slurry A, the temperature is usually from 80° C. to 300°C., an preferably from 100° C. to 200° C. Furthermore, in order toremove the residual solvent from the composite particles, they may bedried at necessity after the rolling-fluidized granulation.

By the above method, the composite particle comprising the electrodeactive material, the electric conductive material, and the binder isobtained. In this composite particle, the electrode active material andthe electric conductive material are bonded by the binder and/or thesoluble resin, and the outer shell portion of the composite particle isformed by bonding the electrode active material and/or the electricconductive material whose weight average particle diameters arecomparatively small, and the core portion of the composite particle isformed by bonding the electrode active material and/or the electricconductive material whose weight average particle diameters arecomparatively large.

The second producing method includes a step of obtaining slurry Bcontaining the electrode active material, the electric conductivematerial, and the binder, and a step of spray-drying the above slurry Bby a pin type atomizer to spray-granulate it.

First, the above electrode active material, the electric conductivematerial, the binder, and soluble resin and other additives at necessityare dispersed or dissolved in a solvent to obtain the slurry B which theabove electrode active material, the electric conductive material, thebinder, and soluble resin and other additives at necessity are dispersedor dissolved.

As the solvent used in order to obtain the slurry B, those same aslisted in the above first producing method may be employed. The amountof the solvent used in preparing the slurry B is such that the solidcontent concentration of the slurry B is usually in the range of 1% to50% by weight, preferably 5% to 50% by weight, and further preferably10% to 30% by weight.

The method or procedure to disperse or dissolve the above electrodeactive material, the electric conductive material, the binder, and thesoluble resin and other additives at necessity in the solvent is notlimited in particular. For example, the method of adding the electrodeactive material, the electric conductive material, the binder, and thesoluble resin in the solvent to mix them; the method of dissolving thesoluble resin in the solvent, and adding the binder (for example, Latex)dispersed in the solvent to mix them, and finally adding the electrodeactive material and the electric conductive material to mix them; themethod of adding the electrode active material and the electricconductive material to the binder dispersed in the solvent, and addingthe soluble resin dissolved in the solvent to mix them, and the like maybe mentioned. As means for mixing, for example, mixer machines such as aball mill, a sand mill, a bead mill, a pigment dispersion machine, acrushing machine, an ultrasonic disperse machine, a homogenizer, aplanetary mixer may be mentioned. The mixing is usually performed in therange of room temperature to 80° C., and for 10 minutes to severalhours.

Next, the above slurry B is spray-dried by a pin type atomizer, andspray-granulation are performed. The spray-drying method is the methodof atomizing the slurry in a hot wind to dry it. The apparatus used forthe spray-drying method is a pin type atomizer. The pin type atomizer isone kind of centrifugal type atomizer using an atomizing board, and inthe atomizing board, plural atomizing rollers are attached detachablybetween the upper and lower attachment disks on a roughly concentriccircle along the circumference thereof. The slurry B is guided from thecenter of the atomizing board, and it adheres to the atomizing rollersby centrifugal force and moves outward on the roller surface, andfinally it separates from the roller surface to atomize it.

Although the temperature of the slurry B to be atomized is usually theroom temperature, it may be warmed to higher than the room temperature.

The hot wind temperature at the moment of spray-drying is usually from80° C. to 250° C., and preferably from 100° C. to 200° C. In thespray-drying method, the method to blow a hot wind is not limited inparticular, and for example, the method in which the hot wind and thedirection of atomizing go in parallel in the transverse direction, themethod in which atomizing is carried out in the drying column top partand the atomized droplet falls with the hot wind, the method in whichthe atomized droplet and the hot wind contact in countercurrent, themethod in which the atomized droplet first goes in parallel with the hotwind, subsequently falls by gravity, and then contact the hot wind incountercurrent, and the like may be mentioned.

Carrying out the spray-drying of the slurry B to remove the solvent inthe slurry can gives the composite particle comprising the electrodeactive material, the electric conductive material, the binder, and thesoluble resin. In this composite particle, the electrode active materialand the electric conductive material are bonded by the binder and/or thesoluble resin, and the outer shell portion of the composite particle isformed by bonding the electrode active material and/or the electricconductive material whose weight average particle diameters arecomparatively small, and the composite particle core portion is formedby bonding the electrode active material and/or the electric conductivematerial whose weight average particle diameters are comparativelylarge.

The electrochemical element electrode material according to the presentinvention comprises the composite particles according to the presentinvention, and it includes an additional binder and other additives atnecessity.

The amount of the composite particles contained in the electrochemicalelement electrode material is usually 50% by weight or more, preferably70% by weight or more, and further preferably 90% by weight or more.

As the additional binder contained if needed into the electrodematerial, those same as ones listed up as the binder used in obtainingthe above composite particle may be mentioned. Since the above compositeparticle has already contained the binder, in preparing the electrodematerial, it is not necessary to add the binder separately, but in orderto increase the bonding force of the composite particles, the binder maybe added when the electrode material is prepared. The amount of thebinder to be added when preparing the electrode material, in the sumtotal with the binder in the composite particles, and to 100 parts byweight of the electrode active material, is in the range of usually0.001 to 50 parts by weight, preferably 0.01 to 20 parts by weight, andfurther preferably 0.1 to 10 parts by weight.

The other additives include forming auxiliary agents such as water andalcohol, and the quantity which does not spoil the effect of the presentinvention may be chosen appropriately and may be applied.

The electrochemical element electrode according to the present inventionis made by laminating an active material layer made of the aboveelectrochemical element electrode material on a collector.

As the material for the collector used in the present invention, forexample, metal, carbon, conductive polymer, and the like may be used,and metal is used preferably. As the metal for the collector, aluminum,platinum, nickel, tantalum, titanium, stainless steel, and other alloyand the like are usually used. Among these, it is preferable to usealuminum or aluminum alloy from the viewpoint of conductivity andvoltage resistance. Moreover, when high voltage resistance is required,the high purity aluminum disclosed in Japanese Unexamined PatentApplication Publication No. 2001-176757 and the like may be usedpreferably. The collector is a film or a sheet, and although thethickness thereof is suitably chosen according to applications, it isusually from 1 μm to 200 μm, preferably from 5 μm to 100 μm, morepreferably from 10 μm to 50 μm.

Although the active material layer may be fabricated by forming theelectrochemical element electrode material into the shape of a sheet,and may be laminated subsequently onto the collector, meanwhile, it ispreferable to fabricate the electrochemical element electrode materialdirectly on the collector, and thereby form the active material layer.As the method of forming the active material layer which comprises theelectrochemical element electrode material, although there are drymolding methods such as a pressure molding method, and wet moldingmethods such as an application method, the dry molding methods arepreferable since the drying process is unnecessary and the producingcost can be reduced. As the dry molding methods, there are the pressuremolding method, the extrusion molding method (referred to also as pasteextrusion) and the like. The pressure molding method is the method wherepressure is given to the electrochemical element electrode material, andthereby the material is made thickness by the re-arrangement,modification, and destruction of the electrode material, to form theactive material layer. The extrusion molding method is the method wherethe electrochemical element electrode material is extruded by anextrusion molding machine into a film shape, a sheet shape and the like,and is the method that can continuously form the active material layeras a long object. Among these, it is preferable to use the pressuremolding method since it can be made by simple equipment. As the pressuremolding method, for example, there are the method where the electrodematerial containing composite particles are supplied to a roll pressuremolding apparatus by a feeder such as a screw feeder, and the activematerial layer is formed (in this method, the active material layers canbe directly laminated on the collector by sending the collector betweenthe roll at the same time when the electrode material is supplied); themethod where the electrode material is spread on the collector, and theelectrode material is leveled by a blade or the like, and its thicknessis adjusted, then it is formed by a pressure molding apparatus; themethod where the electrode material is filled up in cavity of a mold andthe mold is pressurized and formation is made, and so on. It ispreferable that the temperature at the moment of molding is from 0° C.to 200° C.

In order to eliminate uneven thickness of the formed electrode, and toincrease the density of the active material layer and to achieve highcapacitance, post-pressing may be performed further if needed. As themethod of post-pressing, press process by a roll is generally performed.In the roll press process, two cylindrical rolls are arranged verticallyin parallel with each other with a narrow interval, and they are rotatedin opposite directions, and an electrode is inserted into them to bepressurized. The temperature of the roll may be adjusted by heating orcooling.

EXAMPLE

The present invention is explained still more specifically withreference to Examples and Comparative Examples hereafter, however, thepresent invention is not limited to the following Examples. Moreover,part and % are by weight, unless otherwise specified.

Example 1

100 parts of electrode active material (activated carbon having specificsurface area of 2000 m²/g and weight average particle diameter of 5 μm),5 parts of electric conductive material (acetylene black “Denka BlackPowder”: manufactured by Denki Kagaku Kogyo K.K.), 7.5 parts ofdispersible binder (aqueous dispersion of cross-linked acrylate polymerwith average particle diameter of 0.15 μm, and glass transitiontemperature of −40° C.: “AD211”: manufactured by Zeon Corporation), 93.3parts of soluble resin (1.5% aqueous solution of carboxymethyl cellulose“DN-800H”: manufactured by Daicel Chemical. Industries, Ltd.), and 341.3parts of ion exchanged water were stirred and mixed by a TK homomixer toobtain slurry having solid content of 20%. Subsequently, the slurry wascharged into a hopper 51 of a spray drier (with a pin type atomizer,manufactured by Ohkawara Kakohki Co. Ltd.) as shown in FIG. 5, and sentto the top nozzle 57 with pump 52, and sprayed from the nozzle into thedrying column 58. At the same time, 150° C. hot wind was sent into thedrying column 58 from the side of the nozzle 57 through the heatexchanger 55 to obtain a spherical composite particle A-1 havingparticle diameter of 10 μm to 100 μm (average particle diameter of 50μm.) The electron microscope observation image of the obtained compositeparticle was shown in FIG. 6. The composite particle A-1 consisted of acore portion and an outer shell portion, and in the core portion, theshapes of particles having large particle diameter can be found, while,in the outer shell portion, fine particles whose individual shape cannotbe found were bonded mutually. That is, the weight average particlediameter of the electrode active material and the electric conductivematerial which form the outer shell portion, was smaller than the weightaverage particle diameter of the electrode active material and theelectric conductive material which form the core portion.

The obtained composite particle 3, as shown in FIG. 4, was supplied fromthe feeder 4 into the roll 5 (roll temperature 100° C., line pressure3.9 kN/cm) in a roll pressing machine (pushing cut rough surface heatroll; manufactured by Hirano Engineering Research Institute Ltd.), andwas formed on an aluminum collector 1 having thickness of 40 μm atmolding rate 3.0 m/min to give an electrode sheet which has an activematerial layer 2 with thickness of 350 μm, width of 10 cm, and densityof 0.58 g/cm³. The capacitor characteristics of this electrode sheetwere shown in Table 1.

Example 2

Slurry was obtained in the same manner as in the Example 1 except that5.6 parts of polytetrafluoroethylene were used in the place of thebinder (AD-211) used in the Example 1, and by use of this slurry, aspherical composite particle A-2 of particle diameter 5 μm to 70 μm(average particle diameter of 50 μm) was obtained. The compositeparticle A-2 consisted of a core portion and an outer shell portion inthe same manner as the composite particle A-1, and the weight averageparticle diameter of the electrode active material and the electricconductive material which form the outer shell portion, was smaller thanthe weight average particle diameter of the electrode active materialand the electric conductive material which form the core portion, andhad the same structure as that of the particles as shown in FIG. 3.

The obtained composite particle was rolled and formed by use of the rollpressing machine in the same manner in the Example 1, and an electrodesheet having an active material layer which had thickness of 380 μm,width of 10 cm, and density of 0.59 g/cm³ was obtained. The capacitorcharacteristics of this electrode sheet were shown in Table 1.

Example 3

Slurry S1 (solid content 8%) made of 2 parts ot an electric conductivematerial (Denka Black Powder: manufactured by Denki Kagaku Kogyo),7.5parts (solid content 40%) of a binder (AD211: manufactured by ZeonCorporation), 3.33 parts (solid content 4%) of carboxymethyl cellulose(“DN-10L” manutacturedby Daicel Chemical Industries, Ltd.), 17.76 parts(solid content 1.5%) of carboxymethyl cellulose (“DN-800H” manufacturedby Daicel Chemical Industries, Ltd.), and 36.9 parts of ion exchangedwater was prepared.

100 parts of an electrode active material (activated carbon havingspecific surface area of 2000 m²/g and weight average particle diameterof 5 μm) was charged into an Agglomaster manufactured by Hosokawa MicronCorp., and it was fluidized by 80° C. hot wind, and the above slurry S1was atomized into the Agglomaster, and fluidized-granulation wasperformed to give a particle A. The average particle diameter of theparticle A was 40 μm.

Slurry S2 (solid content 10.9%) made of 3 parts of an electricconductive material (Denka Black Powder: manufactured by Denki KagakuKogyo), 0.625 part (solid content 40%) of a binder (AD211: manufacturedby Zeon Corporation), 5.0 parts (solid content 4%) of carboxymethylcellulose (“DN-10L” manufactured by Daicel Chemical Industries, Ltd.),and 26.64 parts (solid content 1.5%) of carboxymethyl cellulose(“DN-800H” manufactured by Daicel Chemical Industries, Ltd.) wasprepared.

The particles A were charged into a rolling-fluidized granulatingmachine (Henschel Mixer), and it was rolling-fluidized granulated whilethe slurry S2 was atomized to obtain composite particle A-3. Thiscomposite particle was spherical and its average particle diameter was50 μm. The composite particle A-3 consisted of a core portion and anouter shell portion, and the weight average particle diameter of theelectrode active material and the electric conductive material whichform the outer shell portion was smaller than the weight averageparticle diameter of the electrode active material and the electricconductive material which form the core portion.

The obtained composite particle A-3 was rolled and formed by use of theroll pressing machine in tho same manner in the Example 1, and anelectrode sheet having an active material layer which had thickness of350 μm, width of 10 cm, and density of 0.57 g/cm³ was obtained. Thecapacitor characteristics of this electrode sheet were shown in Table 1.

Comparative Example 1

The particles A obtained in the intermediate process of the Example 3were particles having a structure where only the electric conductivematerial adhered to the surroundings of the electrode active material,and was not of a 2-layer structure of a core portion and an outer shellportion.

Although this particle A was tried to be rolled and formed by use of theroll pressing machine in the same manner in the Example 1, it was notpossible to perform formation.

Comparative Example 2

In the Comparative Example 1, roll forming was carried out in the samemanner as the Comparative Example 1 except that the line pressure of theroll was changed into 9.8 kN/cm, and the molding rate was changed into0.5 m/min, and an electrode sheet which had an active material layerwith thickness of 320 μm, width of 10 cm, and density of 0.59 g/cm³ wasobtained. The capacitor characteristics of this electrode sheet wereshown in Table 1.

TABLE 1 Electrode Active material Internal density layer thicknessCapacitance resistance [g/cm³] [μm] [F/g] [Ω] Example 1 0.58 350 54.612.2 Example 2 0.59 380 53.4 11.9 Example 3 0.57 350 54.5 9.8Comparative Impossible to form Example 1 Comparative 0.59 320 46.2 13.2Example 2

Evaluation Method of Capacitor Characteristics

(Electrode Density)

An electrode of a size 40 mm×60 mm is cut down from the electrode sheet,the weight and volume of the electrode were measured, and the electrodedensity excluding the collector portion was calculated.

(Capacity and Internal Resistance)

The electrode sheet was punched out to obtain two circular electrodeswith diameter 12 mm. The active material layers of the electrodes wereopposed each other, and a rayon separator of thickness 35 μm wasinserted thereinto. To this, electrolysis solution in whichtriethylmonomethylammonium tetrafluoroborate was dissolved intopropylene carbonate at concentration of 1.5 mol/L was impregnated underdecompression to give a coin cell CR2032 type electric double layercapacitor.

The obtained electric double layer capacitor was used, and it wascharged at 25° C. for 10 minutes at constant current 10 mA from 0V to2.7V, and then to 0V, it was discharged at fixed current 10 mA. From theobtained charge-discharge curve, capacitance was calculated, and it wasdivided by the mass of only the active material layer of the aboveelectrode to give the capacitance per unit mass of the active materiallayer. Moreover, the internal resistance was calculated from thecharge-discharge curve according to the calculation method of thestandard RC-2377 specified by the Japan Electronics and InformationTechnology Industries Association.

As is apparent from the above Examples and Comparative Examples, whenthe electrochemical element electrode material that comprises thecomposite particle according to the present invention that includes acore portion and an outer shell portion, where the weight averageparticle diameter of the electrode active material and the electricconductive material which form the outer shell portion is smaller thanthe weight average particle diameter of the electrode active materialand the electric conductive material which form the core portion, it ispossible to obtain electrodes for electric double layer capacitors witha high electrode density. Moreover, when the obtained electrodes isused, it is possible to produce electric double layer capacitors withlow internal resistance and large electric capacitance.

On the other hand, it is known that with composite particles where largeparticles and small particles are distributed uniformly, and compositeparticles of the structure where only the electric conductive materialis adhered to the electrode active material, the internal resistancedoes not become low enough, but the capacitance is low.

Industrial Applicability

Use of the electrochemical element electrode obtained in this mannerresult in producing an electrochemical element with low internalresistance and high electric capacitance, which allows it to be usedpreferably for various applications including backup power sources ofmemories in personal computers or personal digital assistants, powersources for instantaneous power failure measures in personal computersand the like, applications to electric cars or hybrid cars, solar powergeneration energy storage systems used together with solar cells, loadleveling power sources combined with batteries and the like.

1. A composite particle for electrochemical element electrode,comprising electrode active materials, electric conductive materials andbinders, and being structured of an outer layer portion and an innerlayer portion; wherein the outer layer portion and the inner layerportion are made by bonding the electrode active materials and theelectric conductive materials by the binders, and the weight averageparticle diameter of, the electrode active material and the electricconductive material which form the outer layer portion, is smaller thanthe weight average particle diameter of, the electrode active materialand the electric conductive material which form the inner layer portion.2. The composite particle for electrochemical element electrodeaccording to claim 1, wherein the binders are a dispersible binder. 3.The composite particle for electrochemical element electrode accordingto claim 1, wherein the electrode active materials are activated carbonshaving a specific surface area of 30 m²/g or higher.
 4. The compositeparticle for electrochemical, element electrode according to claim 2,wherein the dispersible binder are acrylate polymers.
 5. The compositeparticle for electrochemical element electrode according to claim 2,wherein the dispersible binders are polytetrafluoroethylene.
 6. Thecomposite particle for electrochemical element electrode according toclaim 2, further comprising a soluble resin.
 7. An electrochemicalelement electrode material comprising the composite particle accordingto claim
 1. 8. An electrochemical element electrode wherein an activematerial layer made of the electrochemical element electrode materialaccording to claim 7 is laminated on a collector.
 9. An electrochemicalelement electrode according to claim 8, which is for an electric doublelayer capacitor.